Hearing-impaired individuals indicate as one of their greatest disabilities the inability to segregate sounds of interest from a complex auditory scene. In normal-hearing listeners, sound segregation is enhanced by localization of the relevant sound source(s), by spatial release from masking, and by spatial stream segregation. The cortical substrates of these three aspects of spatial hearing will be examined using closely coordinated psychophysical studies in humans and cats combined with cortical recording from cats in anesthetized and awake-behaving conditions. Specific Aim 1 tests the hypothesis that the spatial selectivity of cortical neurons is sharpened when a cat is actively engaged in a localization task compared to when it is inattentive. Further, it will test the hypothesis that such task-dependent sharpening of spatial selectivity is enhanced in particular cortical areas that have been shown to be necessary for normal sound localization behavior. Specific Aim 2 will define the cortical mechanisms for spatial release from masking. It will characterize spatial release from energetic masking and will develop a novel animal model for informational masking. It will test the hypothesis that informational masking arises at the level of auditory cortex and that spatial masking release is enhanced in auditory cortical belt areas. Specific Aim 3 develops an animal model for study of spatial stream segregation at the cortical level. It tests the hypothesis that perceptually segregated streams correspond to discrete populations of activated neurons. In addition, it tests the hypothesis that cortical areas that are specialized for spatial stream segregation are distinct from those specialized for sound localization. All three specific aims will discriminate between sub-cortical and cortical mechanisms and will test whether particular aspects of spatial hearing are mediated by distinct cortical areas. This will aid in diagnosis and treatment of spatial hearing disorders. Aims 2 and 3 will isolate and quantify the contribution of specific low- and high-frequency spatial cues to spatial release from masking and to spatial stream segregation. Understanding of the relative importance of low-frequency fine- structure cues and high-frequency sound-level cues will inform design of sound processing schemes intended to enhance hearing in complex auditory scenes by users of hearing aids and cochlear implants.